Linear motors are known in the art. According to a typical configuration, the linear motor includes an armature which makes up the stator. The armature includes a yoke made up of a pack of ferromagnetic laminations. The yoke includes a plurality of teeth arranged at a predefined pitch, with a plurality of slots respectively separating the teeth. The armature further includes coil windings wound around the teeth and housed in the respective slots.
The linear motor also includes a magnet rail which forms the rotor. The magnet rail includes a plurality of plate-like permanent magnets. The magnets are positioned linearly along the rail at a predefined pitch with corresponding gaps therebetween. The armature travels along the length of the magnet rail with the teeth of the armature adjacent the magnets. The position of the armature is determined via a sensor, and a controller controls the current provided to the coil windings based on the armature position. In this manner, the armature may be selectively driven back and forth along the magnet rail. See, e.g., U.S. Pat. No. 5,642,013.
One particular type of linear motor is known as a double-sided linear motor. In a double-sided linear motor, the armature includes a pair of yokes symmetrically disposed on opposite sides of the magnet rail. Each yoke includes its own set of coil windings. The windings of both yokes are driven so as to increase the amount of force available from the linear motor as compared to a more conventional single yoke armature. See, e.g., U.S. Pat. No. 4,868,431.
Linear motors such as those described above are quite useful in a variety of applications. These applications include, but are not limited to, control systems, manufacturing processes, robotics, etc. Linear motors provide precision linear movement in a whole host of applications.
Despite the recognized advantages associated with known linear motors, there have been a number of drawbacks. For example, it is desirable that the double-sided linear motor maintain an air gap of approximately equal dimension between the yoke and the magnet rail on each side of the magnet rail. Failure to provide such equal airgap results in uneven magnetic forces being exerted on the magnet rail. In the case of a relatively thin magnet rail, this can result in a bending of the rail which further exacerbates uneveness in the air gap and the magnetic forces exerted on the magnet rail by the respective yokes.
Still another drawback associated with linear motors is “cogging”. Linear motors have a cogging or detent force that is created by the interaction between the permanent magnets on the magnet rail and the magnetic iron forming the teeth of the armature yoke. Such cogging occurs even when the windings are not energized. Cogging typically occurs at a frequency that is determined by the number of slots per North-South permanent magnet cycle on the magnet rail. There typically are several cycles of this cogging in one North or South magnet pole cycle.
In addition, because the armature in linear motors is not infinitely long it has magnetic ends (in the direction of travel). The magnetic field at the ends of the armature is different from the magnetic field at the interior of the armature. This difference in magnetic fields causes a second cogging or detent force, referred to herein a “end effect cogging”. End effect cogging does not exist in a rotary motor because rotary motors do not have a magnetic end as will be appreciated.
In general, cogging forces introduce disturbance forces into the operation of linear motors. There have been several approaches in the past for reducing such cogging forces. See, e.g., U.S. Pat. Nos. 4,638,192, 4,912,746, 5,744,879 and 5,910,691. However, these approaches have met with only varying degrees of success. Moreover, these approaches oftentimes require significant modifications to both ends of the armature which leads to undesirable complexity, increased manufacturing costs, etc.
Yet another drawback with conventional linear motors is complexity associated with manufacture. The size and length of the armature, for example, is dependent upon the particular application of the motor, the desired force, etc. From the point of view of the manufacturer, this can result in the frequent need to custom manufacture an armature. Alternatively, the manufacturer may need to keep in stock a variety of different size and length armatures.
In view of the aforementioned drawbacks associated with conventional linear motors, it will be appreciated that there is a strong need in the art for a linear motor and method for designing the same which overcomes such drawbacks. More particularly, there is a strong need in the art for a double-sided linear motor which is less susceptible to uneven air gaps and/or bending of the magnet rail. Moreover, there is a strong need in the art for a linear motor that is less susceptible to the detrimental effects of cogging, and particularly those of end-cogging. Furthermore, there is a strong need in the art for a linear motor that is readily and efficiently manufacturable in different lengths without requiring complete custom design.